Liam was readmitted to hospital on Monday for examination and neurological assessment, including a 24-hour scan to measure electrical activity in his brain.

The neurologist leading the medical team have told the boy’s parents that they can plan to return home a week earlier than expected because of his exceptional recovery.

Since the trial began in August 2010, only four other children worldwide have participated in gene treatment.

Following the procedure, Liam astounded staff by sitting up unaided and drinking by himself while watching his favourite cartoon on a portable DVD player.

The family, who have been staying in the hospital’s guest facility, where Liam has been monitored daily for any adverse reactions, now plan to return home on May 29.

There have been striking improvements in Liam’s speech and his general mobility, which may or may not be down to the gene transfer.

Mr Heffernan said they had noticed a big drop in seizures since Liam had underwent the procedure.

“There has been a huge improvement in his speech — he is coming out with new words every day. There has also been a noticeable improvement is his gait — he runs with speed and can turn on a penny,” he said.

“In fact, Liam’s recovery went so well, he was permitted to leave the hospital just two days after the surgery, to enjoy some of the sights of New York, including Central Park and Times Square, although luring the pigeons of Manhattan with bread seems to be his favourite thing.”

The experience has also been heart-wrenching for the Heffernans who wanted the treatment for their daughter. She missed out on the treatment last October because she was not strong enough.

Mr Heffernan said Liam was an ideal candidate for the treatment because he was pre-symptomatic and, if the genes are accepted by his body, doctors believed they would stop the disease’s progression.

“The best we can wish for is that he will have a normal life, of course, but the next best thing would be an extension of life. We were told that if this treatment did not work he would not live past the age of four,” he said.